The present application is related to the co-pending, commonly assigned, application of Katsuhiko Kodama et al, Ser. No. 729,145, filed on Apr. 30, 1985.
This invention relates generally to air/fuel ratio control of an internal combustion engine equipped with exhaust gas recirculation system, and more specifically to apparatus for controlling air/fuel ratio to provide a desired air/fuel ratio irrespective of the change in atmospheric pressure.
Exhaust gas recirculation, which is known as EGR, is used for reducing the concentration of nitrogen oxides (NOx) included in exhaust gasses from an internal combustion engine. In an internal combustion engine equipped with such an EGR system, it has been a difficult problem to effect accurate air/fuel ratio control since the amount of fresh air taken into the engine cylinders cannot be directly detected in so called speed-density system. This is caused from the fact that the intake gas introduced into engine cylinders includes both fresh air and recirculated burnt gasses in an engine with EGR system.
One approach of providing accurate air/fuel ratio control is to correct air/fuel ratio using the intake pressure and rotational speed of the engine as variables as disclosed in a Japanese patent provisional publication No. 48-27130.
However, this prior art technique suffers from a problem that air/fuel ratio is undesirably changed when atmospheric pressure varies. This is because the change in atmospheric pressure results in the change in back pressure and thus, the concentration of air or oxygen of the intake air varies even though intake pressure detected by an intake pressure sensor exhibits the same value. In speed-density system, since the quantity of intake air is estimated using only intake pressure, necessary correction of air/fuel ratio for compensating for the change in air and oxygen concentration due to the change in back pressure cannot be made, and therefore air/fuel ratio is apt to deviate from a desired value.